HIV persists in the face of highly active antiretroviral therapy (HAART) due to constitutive low-level replication in sites that are poorly accessible to drugs and the development of latent infections in a variety of types including the long-lived memory CD4+ T cell population, macrophages, and microglial cells in the brain - cells that play a critical role in neurologic dysfunction and neurotoxicity. The persistence of HIV mandates that patients be maintained on life-long therapy with its attendant costs and side effects. Furthermore, although HAART has greatly extended survival to patients infected with HIV-1, current therapy has failed to decrease the prevalence of HIVneurological diseases especially in individuals who abuse drugs. This proposal focuses on defining the molecular basis for HIV silencing, the impact of drugs of abuse on the creation and reactivation of the latent viral reservoir in microglial cells, and the development of novel therapeutic approaches to attacking HIV latency. Key questions about HIV silencing that remain to be answered include: What are the primary sequence triggers and mechanisms that induce silencing (i.e. protein repressors and/or viralderived RNA)? What conditions lead to proviral DNA methylation? Do similar silencing mechanisms operate in each of the cell types infected by HIV? How do drugs of abuse reactivate HIV? In partial answer to these questions we have demonstrated that distinct epigenetic silencing mechanisms lead to viral persistence in different cell types. Using novel unbiased shRNA screens we have shown that in T-cells, the polycomb repressive complex 2 (PCR2) plays a central role in maintaining proviral latency, whereas in microglial cells, PCR2 is absent and silencing is mediated by the SMRT and CoREST silencing machinery. Because of these differences in the silencing machinery we have been able to identify selective activators of HIV transcription in the two different cell types. Similarly, methamphetamine is able to reverse proviral latency in microglial cells through a novel activation pathway leading to NF-kB mobilization. This mechanism may outline the initial biochemical events leading to the observed increased neurodegeneration in HIV-positive individuals who use METH. Extending this work we plan to develop novel technologies allowing exploitation a natural epigenetic silencing mechanisms involving both histone and DNA methylation, to block HIV transcription.